1. Field of the Invention
The present invention relates to an amorphous polyimide, and more particularly relates to amorphous polyimide powder having good heat stability and processability, and more particularly, to relates a process for preparing the powder, and a heat resistant adhesive and bonding method by use of the powder.
2. Related Art of the Invention
Conventionally, polyimide prepared by reaction of tetracarboxylic acid dianhydride with diamine is excellent in mechanical strengths and dimensional stability in addition to high heat-resistance and also has flame retardance and electrical insulation properties. Consequently, polyimide has been used in the fields such as electric and electronic devices, space and aeronautic equipment and transport machinery, and is expected for wide use in the fields where heat-resistance is required.
Various kinds of polyimide which exhibit excellent properties have conventionally been developed. In particular, polyimide having recurring structural units of the formula (I): ##STR3## has been disclosed by Proger et al. in U.S. Pat. No. 4,065,345 and known to have excellent mechanical, thermal and electrical properties and resistance to solvent and heat and also to exhibit melt flowability. The polyimide, however, has been difficult to process such as by injection molding and extrusion because of high melt viscosity as compared with other common engineering plastics which can be injection molded or extruded.
The present inventors have investigated in order to solve these problems, blocked the reactive terminal of a polymer which essentially has the fundamental structure of the above formula (I) to control the molecular weight of the polymer, and thus developed an injection--and extrusion processable polyimide as disclosed in Japanese Laid-Open Patent Hei 2-018419. The polyimide, however, has a melting point of about 360.degree. C. as reported by Ohta et al. in 3th International SAMPE Symposium, Apr. 2-5, P1030(1990) and thus has a disadvantage that the polyimide must be noncrystallized in order to process or bond the polyimide.
The polyimide of the above formula (I) can be utilized for adhesive having excellent heat resistance and primarily used for bonding metals, prepreg, ceramics and polyimide films for FPC substrate, and is expected for wide use as an adhesive by utilizing the thermoplastic property.
Conventionally, adhesion has been carried out by the following methods.
(1) Varnish of polyamic acid precursor is applied to an adhesive surface, pressed and heated to carry out adhesion by solvent removal and imidization. PA0 (2) A polyimide film is inserted into adhesive surfaces and adhesion is carried out by heating under pressure. PA0 (3) Polyimide powder is suspended in a volatile solvent such as alcohol, applied to an adhesive surface, followed by evaporating the solvent and then adhesion is carried out by heating under pressure.
Many other investigations have been carried out to suspend polyimide powder in a solvent, prepare prepreg by impregnating carbon cloth with the resulting suspension, prepare a composite by using the prepreg and to use the composite as a structural material.
In the adhesion using the conventional crystalline polyimide powder of the above formula (I), adhesion cannot be carried out unless the temperature exceeds the melting point of the polyimide.